1. Field of Invention
The present disclosure relates to electron beam production and, more particularly, to the production of electron beams with various types of phase dislocations in scanning imaging instruments.
2. Description of Related Art
Electron beams may be produced having different phase dislocations in an electron microscope. Phase-dislocated beams may be used to provide additional information about a specimen when used for illumination or as a probe in an electron microscope. This additional information may be due to the different interaction between a phase-dislocated electron beam and sample, when compared to conventional electron beams. One type of phase dislocation is a spiral phase dislocation which may be an azimuthal phase that winds around the optical axis an exact integer of a number of times per wavelength. An electron wavefunction having such a spiral phase dislocation may carry quantized orbital angular momentum and possess an associated magnetic dipole. Electron beams having spiral phase dislocations may be used to form atomic resolution images of magnetization in a sample.
In addition to spiral phase dislocations, other types of dislocations may result in other unique beam-specimen interactions. Electron beams having other types of phase dislocations may provide information about the physical structure, composition, optical, electronic and magnetic properties of a specimen.
Many current forms of electron microscopy illuminate specimens with electron beams that have topologically simple wavefronts with no phase dislocations. The scanned beam microscopes (SEM and STEM) may provide information about a specimen or sample by detecting signals generated by the tightly focused probe beam at the specimen. Phase dislocations in the beam may be avoided in order to provide uniform intensity within the smallest possible focal spot. Alternatively, in TEM the specimen may be uniformly illuminated by a wide beam that has flat electron wavefronts. Post-specimen imaging optics may be used to provide spatially-resolved signals to form images.
Recently, several new TEM techniques have been developed to introduce precise phase dislocations onto the electron beam after a sample. These techniques use microfabricated electron optical elements. Depending upon the type of phase dislocation introduced, and its method of use, these techniques can be used to enhance image contrast or provide entirely new types of image contrast altogether. For example, in order to enhance the visibility of weak phase objects in a TEM, such as unstained biological specimens, phase plates may be used to introduce an artificial phase dislocation between low and high transverse spatial frequency components in the transmitted electron beam. This may provide a high contrast, high resolution image in a TEM specimen that may have been otherwise invisible.
Using these new TEM techniques, the phase imprinting is performed after the specimen in a back focal (Fourier) plane. This represents a drawback that the electron optical elements are placed after a specimen in a TEM, as opposed to before the specimen. This timing difference requires that that the specimen be exposed to a higher beam dose since the electron phase-imprinting elements only imprint the desired phase onto a fraction of the electrons that have passed through a sample.
There is a need for an electron beam phase-imprinting technique that imprints phase dislocations before the specimen.
In addition, commercial phase-imprinting devices may imprint the phase dislocation directly using refraction through a material. Accordingly, the shape and material of the structure may be required to be engineered to extremely high tolerances. Moreover, the structure's shape and material may be required to be kept completely clean from surface contamination since local charging may change the electron optical path length. These drawbacks may have so far prevented the widespread use of phase plate-enhanced electron microscopy.
In at least one implementation of a refraction-based phase-imprinting technique of the prior art, a spiral phase dislocation was imprinted onto an electron beam using refraction through a nanoscale phase plate, which was a membrane having a spiral thickness. A drawback of this technique is that it may only work for particular electron beam energies, and it may be difficult to produce quantized phase dislocations (vortices) with integer helicity.
Another drawback of this technique is that it may not be feasible to create steeply spiraled structures that are capable of generating beams with large amounts of angular momentum per particle. As yet another drawback, only one beam with a particular phase dislocation may be produced, so this technique may be not be able to provide a differential contrast signal, which may be formed by comparing interactions between different beams and the sample. Moreover, this refraction-based spiral phase-imprinting technique demonstrated only the phase imprinting and did not obtain images of other samples. Only the phase plate itself was imaged.
There is a need for a phase-imprinting technique that overcomes the above-mentioned drawbacks.
In at least one other implementation of the prior art, electron beams with spiral phase singularities were produced in an electron microscope using diffractive holograms. This prior art became available after the invention of the present disclosure. This technique demonstrated a magnetic dichroistic signal in a specimen by alternating between beams having spiral dislocations to analyze a different specimen. However, this technique cannot be used to provide high resolution images using this dichroistic signal without significantly modifying the optics of the TEM. In this implementation, the hologram mask was used as a post-specimen image filter in the Fourier plane in a conventional TEM imaging arrangement.
There is a need for an electron phase-imprinting technique that provides high-resolution images.